This invention relates to the field of artificial joint prostheses and, in particular, to an improved instrument for machining a precise cavity in bone for receiving a prosthesis.
For implantation of prosthetic stems, such as hip stems, accurate preparation of the bone or intramedullary canal is extremely important in order to guarantee good contact between the prosthesis stem and the bone. The underlying concept behind precise preparation is that a precise bone envelope reduces the gaps between the implant (i.e. prosthesis or prosthetic component) and the bone, thereby improving the initial and long-term bone ingrowth/fixation. The bone canal is presently prepared for implantation of a prosthetic stem by drilling and reaming a resected end of a bone, such as a femur, and then preparing an area adjacent the drilled hole to provide a seat for the prosthetic stem or a proximal sleeve coupled to the stem of a modular prosthetic system.
Modular prosthetic systems using proximal sleeves, stems, necks and heads, such as the S-ROM Modular Hip System, available from DePuy Orthopaedics, Warsaw, Ind., put more control in the hands of the surgeon, providing solutions for a variety of surgical scenarios, from primary total hip arthroplasty (THA) to the complex revision or DDH challenges. Such system provides such versatility because the provided plurality of stems, sleeves, necks and heads which can be assembled in a large number of configurations.
Preparation of the area adjacent the drilled hole may be accomplished by broaching or by milling. Broaches or rasps, when used for bone preparation, have limitations. One such limitation is the risk of fracture during broaching. Since broaching is done by pounding the broach into the bone, the bone tends to fracture. Additionally, both broaches and rasps suffer from a tendency to be deflected by harder sections of bone so that they do not create as precise a triangular cavity as can be created by a miller system. In a study that compared an intimate fill with robotically machined femora, Paul et al., found that broaching tore the trabecular bone, whereas femoral canal preparation with reamers was consistently more accurate. Paul, H. A., et al. “Development of s Surgical Robot for Cementless Total Hip Arthroplasty.” Clinical Orthopedics and Related Research 285 December 1992: 57-66.
Thus, milling is currently the preferred method of bone preparation in many orthopaedic applications because it is an extremely precise method of bone preparation. A limitation of milling systems today is that they are typically formed so that the drive shaft extends at an angle relative to the remainder of the frame from the end of the miller cutter machining the bone. A fairly large incision must be made to accommodate such milling assemblies. A typical incision for preparing a femur for a total prosthetic hip replacement using a standard triangle miller system is nine inches long. It is not uncommon for incisions as large as 12 inches to be used in a total hip replacement procedure.
A standard triangle miller system typically includes a miller shell, a miller frame and a miller cutter having an end formed for coupling to a drill. A typical miller frame and miller cutter can be seen in U.S. Pat. No. 5,540,694 issued to DeCarlo, Jr. et al. on Jul. 30, 1996. This miller frame allows for precise machining of the triangular canal by a miller cutter held at an angle with respect to the shaft of the frame. The triangular canal facilitates an accurate fit of a proximal sleeve that distributes the load experienced by the prosthesis evenly and provides rotational stability. However, to accommodate this miller, it is necessary to make a fairly large incision which may be undesirable for cosmetic or other reasons.
The large incision is required because the miller cutter includes a fixed unprotected input shaft for connecting to and/or receiving motive (i.e. rotary) power from a drill or similar instrument. As such, the prior reamer is able to accept rotary input power with respect to only one direction. Typically, this direction is at 0° (i.e. “straight on”) with respect to the reamer which is approximately thirty two degrees with respect to the shaft of the miller frame. Therefore, not only is the input power direction restricted, but this, in turn, restricts the angle at which the reamer may be used on a patient. Since the input shaft and the drill coupled thereto extend laterally beyond the edge of the miller frame an incision substantially larger than the width of the frame must be made to accommodate the reamer, frame and drill during surgery. Since the input shaft is unprotected the incision must be large enough to accommodate the reamer, fame, input shaft and drill without the input shaft engaging soft tissue.
In view of the above, it would be desirable to have a bone miller or guided reamer that could fit into a smaller incision during a surgical process.
It would be even further desirable to have a bone milling device as desired above that also is able to accept input rotary power from various angular orientations and/or allows bone milling device to be positioned at various angular orientations relative to the input rotary power.
A milling system is disclosed for creating a cavity in a bone. The cavity has a cross section which has a generally triangular profile having a first side generally parallel with an axis of the bone and a second side forming an acute angle with the first side. The cavity is contiguous with a pre-existing conical cavity in the bone. The system comprises a drive shaft, a frame for carrying a cutter and a cutter for cutting the cavity. The drive shaft has an axis, a proximal end configured for coupling to a drive means and a distal end configured to form a portion of a drive joint for coupling the drive shaft to a cutter. The frame includes a shaft having a longitudinal axis and a cutter mount for mounting a cutter at a first angle approximating the acute angle with respect to the shaft. The mount includes a bracket extending laterally from the shaft to a bearing configured to receive a portion of a cutter and maintain the received cutter oriented at the first angle during rotation. The cutter has a head configured to form a portion of a drive joint for coupling the cutter to a drive shaft. The drive shaft is coupled to the cutter to form the drive joint. The cutter is received in the mount at the first angle and the axis of the drive shaft forms a second angle with the longitudinal axis less than the first angle.
An apparatus for creating a cavity in a bone for receiving a prosthesis which has a conical portion and a projection of a generally triangular profile is disclosed. The apparatus comprises a shell, a frame, and a cutter. The shell includes a conical portion which defines a longitudinal axis and a shaft-receiving cavity for receiving a frame. The frame includes a shaft, a drive shaft and a shield. The shaft is received by the shaft-receiving cavity and is movable with respect to the shell along the longitudinal axis. The frame is configured to carry the cutter disposed at an acute angle relative to the longitudinal axis. The drive shaft is disposed at an angle relative to the longitudinal axis less than the acute angle and is configured at one end to couple to and drive the cutter. The shield is disposed about portions of the drive shaft adjacent the one end. The cutter has a generally triangular profile and is carried by the frame and is configured to mate with and be driven by the drive shaft.
A method for cutting a triangular cavity in bone includes the steps of providing a cutter having a cutter bearing, providing a frame, journaling the cutter bearing in the frame incising the patient, providing a drive shaft and cutting the cavity. The provided cutter comprises a cutting surface having an outer diameter and a cutter bearing at one end of the cutting surface having an outer diameter larger than the outer diameter of the cutting surface. The provided frame has a shaft movable relative to the bone to be prepared which comprises a frame bearing which is complementary to the cutter bearing and which has an inner diameter which is larger than the outer diameter of the cutter's cutting surface and is configured to hold the bearing at a first acute angle relative to the shaft. The shaft has a width extending between the shaft and the bearing.
Journaling the cutter bearing in the frame is accomplished by passing the cutter's cutting surface through the frame bearing and engaging the cutter bearing with the frame bearing. The incising occurs adjacent the bone to be prepared to form an incision having a length approximating the width of the provided frame. The provided drive shaft includes portions configured to couple to and drive the cutter when the drive shaft is disposed at a second angle relative to the shaft which second angle is less than the first angle. The cavity is cut by driving the cutter with the drive shaft disposed at the second angle and moving the frame relative to the bone.
The disclosed triangular miller is configured to reduce the size of incision required for preparation of a bone with a reamer or milling tool to receive a prosthetic stem therein. The disclosed triangular miller frame includes one or more of the following features, either alone or in combination. A drive shaft alignment bracket supporting a drive shaft. A constant velocity joint at the interface between the drive shaft and the milling tool or reamer facilitating various drive shaft angles. A spring loaded sleeve surrounding portions of the drive shaft to protect soft tissue adjacent to the bone being prepared.
The accompanying drawings, which are incorporated in and constitute part of the specification, illustrate the preferred embodiments of the invention, and together with the description, serve to explain the principles of the invention. It is to be understood, of course, that both the drawings and the description are explanatory only and are not restrictive of the invention.
Corresponding reference characters indicate corresponding parts throughout the several views. Like reference characters tend to indicate like parts throughout the several views.